This invention is directed to a process for separating contaminating metal ions from an acid solution containing predominantly iron(II) chloride obtained from dissolution of cyclone dust from the preparation of titanium dioxide by the chloride process in dilute hydrochloric acid, the separation being effected by increasing the pH and separating the metal hydroxide precipitate which forms.
Iron(II) chloride solutions are used as precipitating and flocculating agents in purifying waste water (See, Wassertechnische Information der KRONOS TITAN-GMBH, Ferrofloc.TM.). Large quantities of iron(II) chloride are generated in the preparation of titanium dioxide according to the so-called chloride process. In this process, a titanium- and iron-containing raw material, for example titanium slag or ilmenite, is chlorinated in the presence of coke as a reducing agent in a fluid bed reactor at temperatures of about 1000.degree. C. In addition to titanium tetrachloride, iron(II) chloride is also formed in the process. The iron(II) chloride, together with the water-insoluble solids [principally coke, undecomposed titanium dioxide and silicon dioxide] as well as other metal chlorides, is separated from the gas leaving the reactor in a cyclone connected to the reactor outlet. The separated mixture is referred to as cyclone dust. Known processes for working up such cyclone dust, for example that described in U.S. Pat. No. 3,867,515, are expensive.
By forming the cyclone dust into a suspension in dilute hydrochloric acid and separating the water-insoluble components, a solution is obtained which contains predominantly iron (II) chloride, but which additionally contains aluminum chloride, manganese chloride, magnesium chloride, zirconium chloride and trace elements such as chromium, niobium and vanadium as chlorides. If such an iron(II) chloride solution is used for waste water treatment and sludge conditioning, steps must be taken, due to the unavoidable content of trace elements in the composition of the raw material, to prevent the waste water from becoming contaminated with excessive levels of undesired chemicals which in turn cause problems in disposing of the waste water. The "contaminated" iron(II) chloride solution must therefore be further purified before its use; i.e. chromium, niobium and vanadium ions must be removed from the solution.
Various techniques are known in the art for recovering metal ions. Mitchell et al, U.S. Pat. No. 3,655,344 discloses treating titanium tetrachloride drier residue by leaching with water and then adding an alkaline reagent to the resulting solution to raise the pH to from 4 to 12 and insolubilize vanadium and chromium values. Sato et al., U.S. Pat. No. 3,754,072 describes the recovery of vanadium values from titanium tetrachloride distillation residue by steam treatment, addition of alkali, filtering out insolubles, adjusting the pH to 0.5 to 2.5, and filtering out the resulting precipitate. Friedman, U.S. Pat. No. 4,579,721 discloses recovery of metals such as nickel or vanadium from scrubber waste by leaching with acid solution to dissolve the metals, raising the pH to selectively precipitate a first metal, filtering out the precipitate, thereafter raising the pH again to selectively precipitate another metal which precipitates out at a higher pH than the first metal, and so on. Hartmann et al., U.S. Pat. No. 5,248,497 describes recovery of purified ferrous chloride by two-step cooling of a filtered solution to crystallize substantially pure ferrous chloride. None of these prior documents teaches how to separate undesired metal ions from a contaminated solution of iron(II) chloride obtained by treating cyclone dust from the production of titanium dioxide by the chloride process with hydrochloric acid.
If the pH of a contaminated iron(II) chloride solution obtained by treating cyclone dust with hydrochloric acid is raised above 1, the dissolved metal ions precipitate as hydroxides according to their solubility products. Iron(II) ions will remain in solution up to a pH of above 6. Thus a contaminated ("technical") iron chloride solution theoretically could be transformed via selective precipitation into a neat iron chloride solution. In practice, however, it is not possible to carry out such a procedure. If one adds a neutralizing agent such as, for example, sodium hydroxide, calcium hydroxide, magnesium hydroxide, sodium carbonate or calcium carbonate, to an acid solution of cyclone dust, then metal hydroxide sols are formed which, due to their colloidal structure, are extremely difficult to filter out and are not suitable for disposal by dumping.
Russian patent No. SU 676,559 describes the precipitation of metal hydroxides from a solution which contains undesired heavy metal ions in addition to iron. The precipitation is effected by addition of an 8% sodium hydrogen carbonate solution at 20.degree. C. By increasing the pH to 4.5, the chromium, in particular, should be concentrated in the filter cake. Under the indicated precipitation conditions, the colloidal hydroxides cannot be filtered out at reasonable cost. The patent does not contain any information about the nature of the filter cake or its disposal.
Attempts to separate undesired metal ions, such as chromium, niobium, zirconium and/or vanadium, from an iron(II) chloride solution by precipitating the undesired metal ions as hydroxides usually produce very highly dispersed colloidal suspensions. It is extremely difficult, if not impossible, to separate such precipitated metal hydroxides by filtration sufficiently quickly and completely for practical application. Moreover, the filtration residue that is obtained is in a form which is not suitable for disposal in a landfill.
U.S. Pat. No. 4,765,908 describes a different type of separation of chromium from metal chloride solutions in which bentonire, especially montmorillonite, at least one flocculent and alkali or alkaline earth metal carbonates are used together with zirconium catalysts and at least one polyelectrolyte, via which the heavy metal ions are adsorptively bound and the separation of the filter cake offers no difficulties. The filtration residue is also disposable. However, this process is very expensive because of the use of chemicals, and it also produces an exceptionally large volume of sludge.
In published German patent application No. DE 41 30 808 A1 a process is proposed for separating the undesired ions from an iron(II) chloride solution prepared from cyclone dust through selective crystallization. Although this process may lead to the desired purification of the iron(II) chloride solution, the capital costs and operating costs of this process are quite considerable.
Despite the expenditure of considerable effort in the prior art, there remains a need for an effective and economical process for removing undesired metal ions from a technical iron(II) chloride solution. Moreover, because large amounts of contaminated iron(II) chloride solution are generated in the production of titanium dioxide by the chloride process, a process is needed which can efficiently and effectively handle large volumes of solution, e.g. on the order of 15 m.sup.3 per hour.